The present invention relates to a multiple CCFL current balancing and sensing scheme, and specifically, to a power supply topology to deliver evenly distributed current to each CCFL in a multiple CCFL system.
Fluorescent lamps are used to provide illumination for general lighting purposes. The critical factors in the design of a cold-cathode fluorescent lamp (CCFL) include efficiency, cost, and size. A fluorescent lamp is a low-pressure gas discharge source. The lamp contains mercury vapor at low pressure with a small amount of inert gas. The inner wall of the lamp is coated with fluorescent powder. The discharge generates visible radiation. The CCFL has efficiency in the range of 40 to 60 lumens per watt and the average life of the CCFL lasts for 10,000 hours or more.
FIG. 1 illustrates a conventional single CCFL current sensing scheme. The prior art includes a controller 2 to control the status of the CCFL. A transformer T1 is coupled to the controller 2 and a CCFL is next coupled to the secondary windings of the transformer T1. A feedback loop 4 is connected between the controller 2 and the CCFL. The power provided by the power supply is controlled by the controller to provide current tending to flow into and out of the parasitic capacitance, induction, and resistance. The controller supplies a waveform (rectangular or square) to the transformer""s T1 primary windings for producing a sinusoidal voltage across T1""S secondary windings and all circuitry in parallel with the winding namely CP, C2 and the CCFL. The original drive waveform is converted to a sinusoid by the filtering action of the resonant circuits consisting of the series combination of CS and T1""s primary leakage inductance and T1""s secondary leakage inductance in parallel with the parallel capacitance comprising of CP in series with C2. The sinusoidal waveform is necessary to create a sinusoidal current in the CCFL. In general, the more discrete resonant circuits exist in any given topology, the greater the filtering action and a purer sinusoidal voltage waveform.
The lamp current produced by this sinusoidal voltage is also sinusoidal with no direct current flowing through the lamp, i.e., the average value of the current flowing through the lamp is zero. To maintain constant lamp intensity, the lamp current is sensed and regulated by the controller 2. The lamp intensity is adjusted by the controller using an external signal applied to the controller (brightness input). The signal may be an analog voltage or may take the form of a digital signal. Depending on the controller design, the lamp current may increase with an increase in the externally applied analog voltage or an increased duty cycle of the digital signal. This method of lamp current control is commonly referred to as xe2x80x9cpositive dimmingxe2x80x9d. Conversely, the applied signal may decrease the lamp current with a decrease in input analog voltage or duty cycle. This method of lamp current control is commonly referred to as xe2x80x9cnegative dimmingxe2x80x9d.
During the time when the sinusoidal input is positive with respect to the circuit common (feedback loop 4), current flows through the lamp and is rectified by D1, developing a voltage across R1 and filtered into a somewhat pure DC voltage by C1 for use by the controller. During the negative half-cycle, the lamp current is rectified by D2, completing the full cycle of lamp current flow. The terminal of the CCFL nearest the circuit common is commonly referred to as the xe2x80x9ccold endxe2x80x9d, while the other terminal is known as the xe2x80x9chot endxe2x80x9d. An open-lamp circuit consists of D3, D4, CP and C2. The open-lamp circuit provides protection against hazardous voltage to operating personnel and against failure of the components experiencing high voltage.
The stray wiring leakage has a slight effect on the linearity of the lamp current verse applied voltage. Generally, the issue is not a serious problem in the single lamp circuit, but can cause more serious problems in multiple CCFL schemes. One scheme for controlling multiple CCFLs is shown in U.S. Pat. No. 6,104,146 to John Chou and Yung-Lin Lin, entitled xe2x80x9cBalanced Power Supply Circuit For Multiple Cold-Cathode Fluorescent Lampsxe2x80x9d.
FIG. 2 shows a prior art multiple lamp circuit having two CCFLs in parallel. The system includes a controller 12 a driving circuit 14, a feedback circuit 16 and lamps CCFL1 and CCFL2. A power supply provides a voltage to the driving circuits 14 that is a self-resonating circuit. The currents that flow into the lamps are similar but not equal due to the fact that the resistance of each path is somewhat different. This scheme does not have the capability of current balancing to control the current used to supply each CCFL.
FIG. 3 illustrates another prior art multiple lamp scheme. The system includes a power supply (not shown), a controller 20, CCFL driving circuit, current balancing circuit 24, and at least two CCFLs. The driving circuit consists of two transformers T1, T2 that are connected in series with primary and secondary windings. The transformers are connected to the controller 20 via their primary windings. CCFL lamps are connected in series and a bridge rectifier D2 is coupled to a common node of the lamps to terminate and sense the lamps"" current. Another bridge rectifier D1 in the driving circuit 22 is used to sense lamp-out for either or both lamps. A resistor R1 is placed at the common node of the transformers T1, T2 secondary windings. The resistor R1 is used to address a lamp-out condition. In the two lamp circuit, the transformer secondary winding currents are equal, but the lamp currents are not equal due to the difference between the amount of parasitic current diversion paths in the two lamp circuits. This issue causes a difference in the amount of current available for each lamp and a resultant mismatch. Further, even if both current paths have identical strays and parallel capacitance, the lamps"" operating voltage mismatch causes the CCFL with higher operating voltage to receive less current. The higher voltage causes more current to be diverted into the parallel capacitor and strays, leaving less current to flow through the lamp. The mismatch between the two circuits also causes a lamp current imbalance. An alternative prior art is illustrated in FIG. 4, which is similar to the aforementioned two-lamp scheme. This scheme uses a single transformer with matched secondary windings.